The Chinese Human Rights Defenders (CHRD) network said Chen Wei, a campaigner from Sichuan, had been criminally detained for inciting subversion. The others remain unaccounted for after they were seized on Sunday.

"These five have not returned home after being taken away by the police. Their homes were searched and their computers were confiscated," said Wang Songlian, from the CHRD.

"The fact that Chen has been criminally detained does not bode well for the rest of this group. We are fearing the worst – detentions and then imprisonments for these activists. If so, this will be one of the worst crackdowns on the mainland's activists in recent years."

Campaigners say they have heard of up to four additional criminal detentions around the country, but have so far been unable to confirm them.

The warnings came as fresh calls for demonstrations were posted on the overseas website Boxun, which carried the first demand. The source remains unknown and many dissidents in China appear to have learned of the message via questioning by police. Only a handful of people sought to protest on Sunday.

The new statement called for people to gather each Sunday afternoon at sites in 13 cities for "non-violent non-co-operation".

It added: "We invite every participant to stroll, watch, or even just pretend to pass by. As long as you are present, the authoritarian government will be shaking with fear."

But Nicholas Bequelin, Asia researcher for Human Rights Watch (HRW), warned: "If you are calling for people to demonstrate on the streets in China, you are exposing them to great danger."

He said it was unclear whether the detentions were a response to the online calls, support for Chen Guangcheng, the lawyer under house arrest, or for supporting the Nobel peace prize given to the jailed dissident Liu Xiaobo.

"Maybe they really are nervous or maybe they are using this to settle scores," said Bequelin. "The main driver of the suppression of activists, which has been going on for three or four years, I think has to do with the fact there are more challenges coming from the citizenry."

Security clampdowns are common before the country's major annual political meetings – which begin next week – although they do not normally go this far.

In an appeal for the release of the three lawyers – Teng Biao, Jiang Tianyong and Tang Jitian – HRW said: "The authorities have failed to give any reason or formal notification to their relatives, and all three are believed to be at risk of ill-treatment and torture."

Last year Teng wrote a graphic account of his detention and assault by police. He said one officer told a colleague: "Let's beat him to death and dig a hole to bury him in and be done with it."

Chen Wei, 42, served two years in jail for his part in the Tiananmen Square pro-democracy movement of 1989 and another five years for a subsequent attempt to commemorate the brutal crackdown. He has been involved in human rights activities in south-western Sichuan province for many years and signed Charter 08, the call for democratic reforms co-authored by Liu Xiaobo.

His wife, Wang Xiaoyan, told Reuters that police detained him on Sunday morning and later searched their home, removing two computers and printed materials.

She said she believed Chen had no involvement with protest calls, adding: "We're a long way from everything here."

Ran Yunfei, a well-known blogger who frequently tackles sensitive subjects, was also detained in Sichuan, while activist Gu Chuan was taken from his home in Beijing.

Sichuan police said they did not know about detentions, while calls to the Beijing public security bureau went unanswered.